Communication systems are known which function to provide reliable, remote operation of a utilization device. In such systems, a suitably encoded signal is produced, usually by modulating one or more tones of specified frequency and duration, on an appropriate RF carrier. At the location of the utilization device, the encoded signal is received and demodulated. The tones are then applied to a decoding apparatus which provides an output signal to actuate the utilization device if, and only if, the tones have the specified frequency and duration.
One such system that has found widespread commercial acceptance in the logging industry, in which it is desirable to effect the remote actuation of a whistle or other signalling device, employs an encoded signal commonly identified as "two-tone sequential." In this system, as more completely described in U.S. Pat. No. Re. 27,044, "Two-Tone Remote Control System," Rothenbuhler et al., issued Feb. 2, 1971 to the assignee of the present invention, a first tone of limited duration and of a specified frequency, typically in the audible frequency range, is modulated upon an RF carrier. Immediately following the termination of the first tone, a second tone of a specified frequency different from the frequency of the first tone, but typically in the audible frequency range, and of a duration equal to the desired actuation time of the whistle, is modulated on the RF carrier. At the location of the whistle, any signals having the frequency of the RF carrier are received and demodulated. The demodulated signals are supplied to first and second decoders. The first decoder includes a resonant circuit which is tuned to the nominal frequency of the first tone, and is operative to provide an output signal for a predetermined period of time following detection of a demodulated signal having a frequency within the bandwidth of the resonant circuit. The second decoder also includes a resonant circuit which is tuned to the nominal frequency of the second tone, and provides an output signal to the whistle for the duration of any demodulated signal having a frequency within the bandwidth of its resonant circuit, but only if such a demodulated signal is detected within the predetermined time period established by the output signal from the first decoder. Such "two-tone sequential" systems provide reliable, remote actuation of a utilization device, even in the presence of encoded signals utilized in other communication systems which have the same RF carrier modulated with a single tone at the nominal frequency of either the first or the second times.
Because the decoders in the system described in U.S. Pat. No. Re. 27,044 utilize resonant circuits, it is to be noted that each resonant circuit must be carefully tuned to ensure that its respective decoder will be responsive only to a specified tone. Since the system is often used for considerable periods of time under harsh environmental conditions, the parameters of the resonant circuits may change, necessitating retuning. It is also necessary to retune the resonant circuits to provide different combinations of first and second tones in situations in which two or more such systems are to be used in proximity to each other to effect the remote actuation of two or more, independent utilization devices. Such retuning is of course costly and time consuming and therefore undesirable.
It is therefore an object of this invention to provide an improved tone decoder particularly useful in two-tone sequential communication systems.
It is a further object of this invention to provide such a decoder which does not utilize resonant circuits.
It is yet a further object of this invention to provide such a decoder in which recalibration of the tone to which the decoder is responsive is relatively easy to effect, compared with the decoders of the prior art.
It is another object of this invention to provide such a decoder in which the nominal frequency of the tone to which the decoder is responsive is relatively stable over long periods of time.
It is yet another object of this invention to provide such a decoder which has a sharply defined and stable bandwidth surrounding the nominal frequency to which the decoder is responsive.
It is still another object of this invention to provide such a decoder which can be implemented utilizing readily available solid-state components and integrated circuits, and which accordingly is inexpensive to manufacture.